To some degree, wall damage in the interior of cavities can be examined optically by means of mirror systems or glass-fiber optical systems, by working through the open end of the cavity. However, this does not provide adequately determined information in many cases; the depth of depressions cannot be accurately determined by optical inspection. Wall damage on the interior of a cavity can, of course, be inspected in detail, by cutting the part forming the cavity, to expose the wall surface. Such destructive testing is normally undesirable and is somewhat impossible.
Non-destructive inspection is, generally speaking, impossible in the case of a heat-exchanger tube. For example, it is necessary to provide for non-destructive inspection of possible damage in a suspected zone of a heat-exchanger tube in the case of a pressurized-water reactor steam generator. At the same time, an accurate determination of the possibility or extent of internal wall damage is important because should the damage warrant, the tube must be plugged and put safely out of service, but the tube should not be put out of service unless the damage makes this a necessity, because the efficiency of the steam generator is lowered by the loss of the tube.
In the above case, it is very desirable to be able to not only make a positive determination of the existence of damage, but also to be able to see its character and to measure the extent and depth of depressions, cracks and the like.